Generally, flexible circuit boards are light, soft, thin, small, ductile, flexible, and supporting high wiring density. Flexible circuit boards can be three-dimensionally wired and shaped according to space limitations. Flexible circuits are generally useful for electronic packages where flexibility, weight control and the like are important.
FIG. 10 illustrates a high frequency signal line structure 100 of a conventional flexible circuit board. The high frequency signal line structure 100 includes two grounding pattern layers 110, a signal line 120 and two medium layers 130 and a plurality of conductive holes 150. The two medium layers 130 entirely and directly contact and enclose the signal line 120, the signal line 120 is sandwiched between the two medium layers 130.
FIG. 11 illustrates that the plurality of conductive holes 150 are evenly distributed at two opposite sides of the signal line 120 and electrically connect the two grounding pattern layers 110. In other words, the grounding pattern layers 110, the plurality of conductive holes 150 are located around the signal line 120, to thereby make a construction of the high frequency signal line structure 100 like a coaxial cable.
A total loss of high frequency signals passing through the signal line 120 is directly determined by line width, impedance of the signal line 120 and a needed signal transmission frequency. The total loss includes loss from the signal line 120 and loss from the medium layers 130. When the signal transmission frequency of the signal line 120 increasing, the loss from the medium layers 130 is greater than the loss from the signal line 120, namely the loss from the medium layers 130 becomes a main factor affecting the total loss of the high frequency signals passing through the signal line 120. Therefore, in order to effectively reduce the total loss, how to reduce dielectric constant of the medium layers 130 is a difficult for the industry to face.
A typical material of the medium layers 130 is selected from teflon, liquid crystal polymer or pure glue with low dielectric constant, in order to reduce the loss from the medium layers 130. Teflon or liquid crystal polymer, however, still has a high dielectric constant, for example, the dielectric constant of teflon is 2.1 farad/meter (F/m), the dielectric constant of liquid crystal polymer is 3.2 farad/meter (F/m). In addition, teflon and liquid crystal polymers are special material, and have high cost, even are monopolized by the main material suppliers.